1. Field of the Invention
The present invention relates to a photo mask delivery box and more specifically, to a photo mask retaining structure for use in SMIF system. The photo mask delivery box holds the photo mask firmly in place against displacement, preventing the generation of particles to reduce contamination levels.
2. Description of the Related Art
Following fast development of high technology, a variety of high-tech products have been developed and intensively used in our daily life, bringing a great convenience to human beings. It is necessary to employ new knowledge and techniques to control high-tech products such as computer, new generation TV, digital stereo system, central control of building, advanced car, aircraft, and other clothing, food, housing and transportation related products. These high-tech products use semiconductor chips obtained from wafers. A photo mask has precision electronic patterns, which is designed to be imprinted on to wafers. During fabrication, a delivery box is used to transport photo mask in the clean room for high precision processing.
Further, the novel standardized mechanical interface (SMIF) system is utilized in a clean room for the fabrication of ICs, which reduces particle contamination by significantly reducing particle fluxes onto wafers. During transportation, storage, and most processing steps, the gaseous media surrounding the photo mask is essentially stationary relative to the photo mask, and particles from exterior environments cannot enter the photo mask environments. The SMIF system of photo mask handling reduces photo mask particle contamination. In order to prevent damage to photo masks, photo mask delivery boxes are used to deliver photo masks. During delivery, photo masks must be maintained stable under a clean environment. However, photo mask delivery box may contain a certain amount of particles or impurities that are not visible to eyes. Therefore, photo mask delivery boxes must be regularly cleaned, ensuring a particle-free status. A minor particle contamination to the circuit pattern on a photo mask may cause a photo failure, resulting in a big yield loss. Because the semiconductor factory and wafer foundry use a big amount of photo masks, the requirement for photo mask delivery box is heavy. However, it is necessary to demount the photo mask delivery box before cleaning. Therefore, it is a heavy work to clean a big amount of photo mask delivery boxes regularly. Further, because a photo mask is an important and expensive device for the fabrication of wafers, damage to the photo masks during cleaning of photo mask delivery boxes delays the fabrication of wafers and relatively increases the manufacturing cost of wafers. A photo mask delivery box is generally made of anti-static material or the like to prevent the storage photo masks or wafers against electrostatic charge. When the anti-static material starts to deteriorate, it must be immediately replaced.
FIG. 8 shows a conventional photo mask delivery box. According to this design, the photo mask delivery box A has top and bottom locating rods A1 for stopping at the top and bottom sides of the storage photo mask B around the border area to hold the photo mask B inside the box A. According to this design, the locating rods A1 may wear or deform quickly with use, lowering the holding force. In this case, the storage photo mask B may be moved forwards and backwards and rubbed against the locating rods A1 during delivery of the photo mask delivery box A, thereby causing a big amount of airborne particles that increase contamination levels.
FIGS. 9 and 10 show another design of photo mask delivery box according to the prior art. According to this design, U-shaped holding blocks D are symmetrically located in the four corners inside the top and bottom cover shells of the photo mask delivery box C. When the photo mask delivery box C is closed, the U-shaped holding blocks D are respectively stopped against the four corners of the storage photo mask B from the top and bottom sides to hold the storage photo mask B in place. This design of photo mask delivery box C still cannot absolutely eliminate displacement of the storage photo mask B relative to the U-shaped holding blocks D during delivery of the photo mask delivery box C, and friction between the storage photo mask B and the U-shaped holding blocks D will generate particles, increasing contamination levels.
Therefore, it is desirable to provide a photo mask delivery box that holds the storage photo mask firmly in place against displacement, resulting from the friction between the storage photo mask and the internal parts of the photo mask delivery box and the generation of preventing particles.